Leetcode-145：二叉树的后序遍历

递归方法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root==null){
            return res;
        }
        postorder(root,res);
        return res;
    }
    
    public void postorder(TreeNode root, List<Integer> res){
        if(root==null)
            return;
        
        if(root.left!=null)
            postorder(root.left,res);
        if(root.right!=null)
            postorder(root.right,res);
        res.add(root.val);
    }
}

非递归方法：

设置两个标记。只有当上一个访问的节点是右孩子时，才访问根节点。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        TreeNode prev = null;
    
        while (!stack.isEmpty() || p != null) {
            while (p != null) {
                stack.push(p);
                p = p.left;
            }
            p = stack.peek();
            // 已经访问过了
            if (p.right == null || p.right == prev) {
                stack.pop();
                res.add(p.val);
                prev = p;
                p = null;
            } else {
                p = p.right;
            }
        }
        return res;
    }
}

第二种非递归遍历方法：
借助一个LinkedList，每次访问栈顶节点后采用头插法插入，从而保证左右根的访问顺序。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> res = new LinkedList<>();
        if (root == null) {
            return res;
        }
        TreeNode p = root;
        stack.push(p);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            res.add(0, node.val);
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
            
        }
        return res;
    }
}